How to install a LED on AC volt.

[paul.1911]

I've a bike where a 35W Head light bulb glow on AC from alternator.
There is High Beam pilot bulb indicator on my meter which glow along with head light on AC
means both get a same power from same source switch.
This AC volts fluctuate with acceleration about minimum form 6-7 i think to 12v max (regulated).

Now, i want to change a meter high beam indicator to LED.
But led needs a constant DC power. Connecting with a battery will not work.
Only I can do is to convert the AC to DC for only one LED.

How can I do that?
Both bulb shares a ground so i can say i've a single wire with fluctuated AC volt.
I want a led to be a constant glow.
Do I need a bridge rectifier? or a single diode will do the work?
Also what rating capacitor do I need?
and a 1k resistor for sure.

Please give me schematic.

Thanks

 

[MikeMl]

I doubt that your motor bike powers anything with AC. All I have seen have an AC alternator, followed by a full-wave bridge rectifier where the pulsating DC output from the rectifier is smoothed by the lead-acid battery. There is also usually a shunt regulator across the battery to hold the battery voltage at ~7V (6V system) or ~14V (12V system).

 

[paul.1911]

I doubt that your motor bike powers anything with AC. All I have seen have an AC alternator, followed by a full-wave bridge rectifier where the pulsating DC output from the rectifier is smoothed by the lead-acid battery. There is also usually a shunt regulator across the battery to hold the battery voltage at ~7V (6V system) or ~14V (12V system).

You are right full wave rectifier are used but in my case rectifier is only used to charge the battery its 2.5A 12v (very small ) and only head light used run on AC which is regulated to 12v.
Rest of the things like horn, indicators, back light, parking lights run on DC via battery.
I'm sure I've a diagram, and therefore at lowest rpm the head light glows like a candle.

 

[paul.1911]

The problem here is nothing. all i want is to convert AC to DC for only one led
I want to know how many diodes do i need? and what rate of capacitor to stop variations?

 

[MikeMl]

Here you go: This puts ~18mA average through the LED. R1 should be rated for 1/2W.

 

[Mr RB]

I doubt that your motor bike powers anything with AC. All I have seen have an AC alternator, followed by a full-wave bridge rectifier where the pulsating DC output from the rectifier is smoothed by the lead-acid battery. There is also usually a shunt regulator across the battery to hold the battery voltage at ~7V (6V system) or ~14V (12V system).

Hi Mike, what you have said is true for all larger and more expensive road bikes, but on small cheap bikes like some scooters and many small off-road bikes the headlight is driven straight from the alternator, and sometimes there can even be no battery.

 

[paul.1911]

Here you go: This puts ~18mA average through the LED. R1 should be rated for 1/2W.

It worked thanks!
I Used IN4007 diode
1k 0.5w resistor
and 1000u 63v capacitor is working fine.